Constructed wetland treatment technology is presently being used world-wide for the efficient and cost-effective treatment of a variety of wastewater effluents Present applications include treatment of municipal and industrial wastewaters, urban stormwater, acid mine drainage (AMD), landfill leachate and agricultural runoff However, as an emerging technology, much remains to be done to enhance performance and provide long-term and reliable operation As noted in Roux, "Constructed Wetlands", informational brochure, 2 pages, 1996, two categories of constructed wetlands treatment technology are currently recognized These are Free Water Surface (FWS) Systems which are similar to a natural marsh, and Subsurface Flow (SSF) Systems which are constructed in excavated and lined trenches back filled with an organic or inorganic treatment media Treatment in an subsurface flow (SSF) system occurs within the media matrix below the surface of the bed. The subsurface flow (SSF) type systems are generally much more efficient, require less space, do not present odor or vector problems and are not subject to freezing They are also aesthetically pleasing Present disadvantages of subsurface flow (SSF) type systems include limited dissolved oxygen transport to the subsurface environment and sludge accumulation and clogging of the treatment media matrix The combination of these factors has resulted in reduced and erratic system performance
The prior art is replete with inventions relating to the treatment of wastewater effluents. The following United States patents are not directly related to the use of constructed wetlands but deal with a variety of problems related to wastewater treatment Mulder et al. U.S. Pat. No. (5,518,618) discloses a system and process for purifying wastewater which contains nitrogenous compounds. Using an apparatus with a first aerated chamber and a second oxygen depleted chamber, wastewater is treated with a sludge containing microorganisms. Stuth U.S. Pat. No. (5,200,081) describes a secondary sewage treatment system using a tank with buoyant media to be interposed between a septic system and a drainfield. Stuth '081 uses aerators to recirculate wastewater to mixing zones, where the wastewater is mixed with the buoyant media in a turbulent fashion. Yamamoto et al. U.S. Pat. No. (5,336,402) relates to a sewage treatment apparatus consisting of an aeration chamber and a settling chamber into which aerated water is introduced from the aeration chamber for sedimentation of sludges. The second chamber has a hopper unit for the discharge of sludge. Vigneri U.S. Pat. No. (5,520,483) discloses a method and system for remediation of hydrocarbon-contaminated subterranean body of groundwater. It involves the use of a plurality of wells to induce the desired chemical reaction of acetic acid and hydrogen peroxide resulting in the generation of hydroxyl free radicals for oxidizing the contaminants. Scraggs et al. U.S. Pat. No. (5,411,665) describes the use of a multi-stage, recirculating series of treatment tanks with oil-consuming microorganisms for reducing and separating emulsions and homogeneous components from contaminated water. Semprini et al. U.S. Pat. No. (5,302,286) describes a method and apparatus for in situ groundwater remediation. Using a well with a venturi section, recirculated water is passed through permeable sections where aeration as well as gaseous or liquid biological nutrients are added.
Herkenberg U.S. Pat. No. (5,451,325) discloses the use of oleophilic substrates to absorb oil from off-shore oil slicks. Winkler U.S. Pat. No. (5,378,355) describes the use of periodically spaced aeration diffusers placed along a linearly extending air distributor conduit. The air diffusers are used to treat wastewater, as well as for a method for in-situ cleaning of the diffusers by supplying liquid or gaseous cleaning fluids to the diffusers The wastewater is treated in a tank.
McGuire U.S. Pat. No. (5,407,572) describes a tertiary wastewater treatment facility using adsorption and filtration. Losack U.S. Pat. No. (5,368,411) discloses a treatment method for treating oil contaminated soil by breaking up the soil, and mixing it with water and a detergent to produce a slurry, and ultimately separating the cleaned soil from the slurry.
Saab U.S. Pat. No. (4,952,315) teaches the use of a method to treat hydrocarbon contaminated water or soil with emulsifiers and bacteria in a closed tank environment Brown U.S. Pat. No. (4,591,443) discloses oxidation of subterranean contaminants from leaking tanks. The oxidation agents are injected directly into the subterranean environment
Ehrlich U.S. Pat. No. (5,549,828) describes in-situ treatment of water bodies by means of an artificial, submerged chamber having air diffusers for forcefully guiding water through filtering media and through rafted or floating vascular plants. Similarly, Hondulas U.S. Pat. No. (5,337,516) discloses treating polluted water using wetland plants in a confined, floating environment
More closely related to constructed wetlands, the following three patents are concerned with various aspects of wastewater treatment.
Grounds U.S. Pat. No. (5,554,289) discloses an apparatus and method for increasing the capacity of a wastewater pond treatment facility including the use of a porous baffle placed in the pond so as to divide the pond into two sections. In the first section, biological solids are treated with aeration and the use of concentrated bacteriological growth, while the second portion is used to accommodate high rates of wastewater flows from storms A microprocessor is used to control the use of various electromechanical subsystems
Horsley et al. U.S. Pat. No. (5,549,817) describes a stormwater treatment system/apparatus designed to collect the first flush of stormwater runoff (which contains the majority of pollutants) and to treat this effluent using sedimentation, filtration and constructed wetlands The apparatus is designed as a self-contained integrated module amenable to mass productions It includes the sedimentation tank surrounded by a ring shaped tank which houses the subsurface flow (SSF) type constructed wetland Multiple modules can be used for larger capacity installations
Morrison et al. U.S. Pat. No. (5,156,741) discloses a wastewater treatment system and method incorporating a subsurface flow (SSF) type constructed wetland utilizing turf grass as the vegetation for bioremediation of secondary treated municipal wastewater. A recirculatory system is used for the subsurface wastewater along with aeration of the subsurface water throughout the bed to enhance turf grass root growth down into the treatment media
However, the prior art does not provide a remedy for the general problems of up-front sludge removal and passive nutrient addition in subsurface flow (SSF) type constructed wetlands